The present invention relates to a starter device which transmits rotational torque to an engine in a motor vehicle for starting thereof.
A starter device for a motor vehicle is constituted in such a manner that its motor unit transmits rotational movement to the pinion gear supported on the pinion shaft and as well its electromagnetic push-out unit displaces the pinion gear. Through the displacement of the pinion gear, the pinion gear couples the ring gear of an engine in a motor vehicle, thereby the rotational movement of the motor unit is transmitted to the engine.
In a conventional starter device, the pinion shaft, on one hand, was arranged on substantially the same axis as that of the motor unit and, on the other hand, the electromagnetic push-out unit was arranged parallel with the motor unit. Thus the electromagnetic push-out unit displaces the pinion gear via a shaft lever. For this reason, the enitre structure of this kind of starter device was awkward and its installation space in the engine room was limited so that the proper arrangement thereof in the engine room was one of difficult problems, further such structure could not meet the current demmand for a high density installation of the motor vehicle components in the engine room.
For solvent the above problem and fulfilling the above demmand, so called starter device with a common axis type electromagnetic push-out unit was proposed wherein the electromagnetic push-out unit was constructed in a cylindrical shape and was concentrically disposed around the circumference of a pinion shaft. One of the examples of these starter devices is disclosed in JP-A-61-85574 (1986).
The above starter device directly transmits the displacement of the electromagnetic push-out unit to the pinion gear without using a shift lever. Therefore, the displacement of the pinion gear with respect to the displacement of the electromagnetic push-out can not be adjusted such as by pivotally supporting the center of the shift lever.
Generally, the electromagnetic push-out unit includes therein a magnetic circuit wherein the displacement caused by the electromagnetic push-out is obtained by magnetic traction force of the movable core toward the stationary core. However, as explained above, when the displacement of the pinion gear could not be adjusted with respect to the displacement of the electromagnetic push-out unit, the amount of displacement of the movable core could not be reduced, therefore a long magnetic resistance portion had to be included in the magnetic circuit in the electromagnetic push-out unit. For this reason, the magnetic field generated by the electromagnetic coil portion in the electromagnetic push-out unit had to be increased to thereby enlarging the size of the electromagnetic coil and as a result the entire size of the starter device was accordingly enlarged.
The magnetic traction force acted upon between the stationary core and the movable core varies dependent upon the square of their gap variation such that the magnetic traction force exponentially increases as the movable core is attracted by the stationary core and approaches thereto. On one hand, the movable core is energized by a return spring so as to return the movable core to the initial position when the current supply from the electromagnetic coil is interrupted. Herein, the energizing force of the return spring varies in proportion to the gap variation between the movable core and the stationary core.
In the starter device with a so called coaxial type electromagnetic push-out unit, since the the distance of displacement of the movable core was increased as indicated above, thereby the gap between the movable core and the stationary core was also increased so that the magnetic traction force was rather increased relative to the return spring force. For this reason, the mechanical strength of the respective mechanical elements in the electromagnetic push-out unit had to be increased and further the structure thereof was complexed which increased the size of the starter device itself.